In the past, it has been conventional practice to include in molding and extrusion grade polypropylene various additives which give heat stability to the polymer. Normally, about 0.5% by weight of the additives is used. The additives include antioxidants such as dilaurylthiodiporpionate, Irganox 1010 (tetrakis[methylene(3,5-di-tert-butyl-4-hydroxyhydrocinnamate)methane]), and Naugard PHR (tris-[mixed mono- and dinonylphenyl]phosphite), sometimes in a mixture. It has now been found that the quantity of such antioxidants may be reduced by 50% or more, by the incorporation into the polypropylene, of a small amount of a less expensive polyester.